In general, in a hybrid circuit, which is used in the case where digital data are transmitted by use of a signal cable for bidirectional communications, a transmission signal and a reception signal are separated from each other by means of a high-frequency transformer. Moreover, when a signal is transmitted through a cable, the signal waveform deteriorates considerably. Therefore, a digital signal processing (DSP) technique has been conventionally used in order to obtain accurate reception data.
In a conventional digital data transmission apparatus as described above, the highest communication speed is 250 mega bits per sec per communication channel, as in the case of a 1000 mega-bit ethernet as disclosed in the following Non-Patent Document 1. Therefore, high-speed data transmission of 1 giga bits per sec per communication channel or higher has been impossible.
The reasons for the impossibility of high-speed data transmission of 1 giga bits per sec or higher are as follows. That is, at frequencies of signals used for high-speed communications, signal separation by use of a high-frequency transformer is very difficult; and an analog-digital converter (A/D converter) and a digital processing circuit, which are used for digital signal processing (DSP), are not practical in terms of operation speed, circuit size, power consumption, and cost.    Non-Patent Document 1: IEEE802.3ab Specifications http://grouper.ieee.org/groups/802/3/ab/